In a power semiconductor device such as an insulated gate bipolar transistor (IGBT) that switches a high current, a high voltage of 10 V or more needs to be applied to a gate of the power semiconductor device. For this reason, the gate of the power semiconductor device is generally controlled by a high side transistor and a low side transistor.
When the high side transistor and the low side transistor are turned on at the same timing, a through current flows between a power supply voltage node and a ground node, resulting in power loss. Therefore, control is required so that the high side transistor and the low side transistor are not simultaneously turned on.
As an example of the control for preventing the high side transistor and the low side transistor from being simultaneously turned on, a gate voltage of the high side transistor and a gate voltage of the low side transistor are monitored. That is, in a case of turning on the high side transistor, first, the low side transistor is turned off and it is confirmed by the gate voltage of the low side transistor that the low side transistor has been turned off, a logical product operation is performed on a low side turn-off signal and a high side turn-on signal, and the high side turn-on signal is transmitted to the high side transistor by a boost level shift circuit to turn on the high side transistor. In a case of turning on the low side transistor, first, the high side transistor is turned off and it is confirmed by the gate voltage of the high side transistor that the high side transistor has been turned off, a signal is transmitted to the low side transistor by a buck level shift circuit, and a logical product operation is performed on the signal and a high side turn-off signal to turn off the low side transistor.
However, at the time of supplying power of a high side power supply VCC, at the time of blocking the supply of the power, or the like, a power supply voltage is temporarily decreased, and thus, there is a possibility that the two level shift circuits described above will not be normally operated and monitor signals cannot be correctly transmitted. When the power of VCC is rapidly decreased in a state where the high side transistor is turned on, in a case where there is no monitoring circuit for a potential difference (VCC-Hs_GND) of a high side floating power supply, if a low side turn-on signal is input, there is a possibility that a high side turn-off monitoring signal will not be stabilized, and the high side transistor and the low side transistor will be turned on, such that a through current will flow.
In addition, it is assumed that there is a monitoring circuit for the potential difference (VCC-Hs_GND) of the high side floating power supply, such that the high side transistor can be turned off. When the buck level shift circuit transmitting a turn-off state of the high side transistor to the low side transistor is not operated, the low side transistor is maintained in a turn-off state, and an output terminal OUT for driving an external transistor becomes a high impedance. If the external transistor is a metal oxide semiconductor (MOS) gate transistor, a gate voltage of the external transistor is not stabilized and an uncontrollable current flows. In order to avoid such a problem, another system that lowers OUT by a monitoring signal between VCC and GND is required. Two systems between VCC and Hs_GND and between VCC and GND are required for monitoring the power supply.
Further, in an application of a switching power supply, the power supply is temporarily interrupted for several microseconds due to a lightning surge, such that there is a demand that a turn-on state of the high side transistor is maintained for a certain time under conditions such as VCC of about 2.4 V and a low temperature of −45° C., and the high side transistor is then turned off. When the buck level shift circuit is not stabilized, the turn-on state of the high side transistor cannot be maintained. Therefore, it is necessary to secure the potential difference (VCC-Hs_GND) of the high side floating power supply even though the power supply voltage is low (2.4 V<VCC<4 V) and a temperature is low (−45° C.).